environment. Scientists have been looking for evidence of why dinosaurs went extinct over
fairly short periods. Many scientists are examining the theory that a major cataclysmic
events, such as an asteroid colliding with Earth, may have caused the extinction of dinosaurs
65 million years ago (Figure7.36).
Figure 7.36: The fossil of Tarbosaurus, one of the land dinosaurs that went extinct during
one of the mass extinctions. ( 21 )
Since life began on Earth, there have been several major mass extinctions. If you look closely
at the geological time scale, you will find that at least five major massive extinctions have
occurred in the past 540 million years. In each mass extinction, over 50% of animal species
died. The total number of extinctions could be as high as 20 mass extinctions during this
period.
The fossil record tells the story of these mass extinctions: millions of species of fish, amphib-
ians, reptiles, birds, mammals, mosses, ferns, conifers, flowering plants, and fungi populated
the seas and covered the Earth - as continents crashed together and broke apart, glaciers
advanced and retreated, and meteors struck, causing massive extinctions. Two specific ex-
tinctions occurred at the end of the Permian period and when the dinosaurs went extinct.
At the end of the Permian, an estimated 99.5% of individual organisms perished. Several
factors may have contributed, and one factor relates again to the supercontinent Pangaea.
Marine biodiversity is greatest in shallow coastal areas. A single continent has a much
smaller shoreline than multiple continents of the same size. Perhaps this smaller shoreline
contributed to the dramatic loss of species, for up to 95% of marine species perished, com-
pared to “only” 70% of land species. Although the exact cause remains unknown, fossils
clearly document the fact of Earth’s most devastating extinction.